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 Post subject: Confusion reigns
PostPosted: Sun Oct 26, 2008 9:34 pm 
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To use the vernacular, these figures on electricity supply and consumption "do your head in" - what with kilo- mega- and gigawatts as units of capacity and megawatt hours, etc., as a units of production. Then, as everybody knows – when you are trying to work out delivered capacity – you take the former, multiply the figures by 8,760 to give the latter and then divide by the load factors. Simple really.

View full article here

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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 12:45 am 
It's yet more complicated than that.

Quite apart from the complexities of trying to match supply and demand at any given time, the Impedance of the entire network has to be carefully maintained and balanced continuously, otherwise bad things happen.

One way of thinking of the UK grid is as a bigger version of a simple school electrical circuit with a battery and a light bulb. I suspect this is quite easy for civil servants in the environmental depratments. Make the no of batteries = the number of lightbuilbs, and all is fine.

However, the wires are much longer than the wavelength of the 50 Hz AC current, hence the wires are 'transmission lines', and these transmission lines have to be terminated with the correct impedances. Electrical engineers working at radio frequencies also have to deal with this, because even though their circuit boards are small, the frequencies used are so high (GHz) that they have to take into account distances of mm on their circuit boards. The 'school electrical set' assumption doesn't hold. This is something that doesn't occur with those simple DC lightbulb circuits. Look at a map of the UK. Why is Hinkley point where it is in that remote SW area? To act as the impedance match in the SW for the entire UK electricity grid!

Of course impedance balancing and load balancing are to some extent two sides of the same coin, but I cannot for the life of me think how they are going to manage it whilst fragmenting the UK electricity supply from a few controllable and predictable sources, to thousands of uncontrollable ones.

One rather concerned Elec Eng here!


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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 12:48 am 
Small addition;

For the sake of rigour, I suppose we should say "Total power from battery should balance the total power dissipated in the lightbulbs"


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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 7:41 am 

Joined: Fri Feb 23, 2007 9:49 am
Posts: 307
passion_chorale wrote:
... the Impedance of the entire network has to be carefully maintained and balanced continuously, otherwise bad things happen. ... Look at a map of the UK. Why is Hinkley point where it is in that remote SW area? To act as the impedance match in the SW for the entire UK electricity grid! ... One rather concerned Elec Eng here!


Do you have a reference for this, or a fuller explanation, perhaps even for those without a knowledge of electrical engineering?

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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 1:00 pm 

Joined: Wed Mar 14, 2007 11:26 am
Posts: 1291
Location: UK
RAENORTH wrote:
and megawatt hours, etc., as a units of production. Then, .... multiply the figures by 8,760

To be pedantic – megawatt hours per annum. ( Pedantry may reduce confusion ? – Well that’s my excuse, anyway )


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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 3:26 pm 
According to the official numbers of issues of ROC's against production claims (which supposedly are claimable against metered outputs on a monthly basis although how one can get claims for 110% of rated capacity I am not sure but it seems you can, so I have my doubts about how the data can be used ...) SOME of the OFFSHORE wind farm SOMETIMES produce up to 50% of their rated capacity. But not too often. ONSHORE do not get close, except one or two places in Scotland once or twice over the last two years.

However the claims, and therefore presumably the actual production levels, are extremely variable within a month to month pattern that seems to be generally observable across all generating sites per type - type being on- or off- shore. One would like to think that the variation is simply the 'wind cull' centres being used as rapid response tools to balance the grid slightly. Or because there is no point in taking their expensive output at certain times. Unfortunately we seem to have 'Denmark Syndrome' - the problem of a distinct lack of output in some months - notably February.

Now since the farms can't produce in the absence of wind or when there is an excess, I assume February fits one of those scenarios - most likely the absence of wind if I recall correctly, due to high pressure systems that also bring clear skies, and cold nights. The same problem that Denmark has reported in the past.

So the question is - how much spare capacity do they have in France and can they get enough of it across the channel when we need it. And at what price?


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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 7:02 pm 
Hi, Bert,

This link is quite a good starting point:

http://www.nationalgrid.com/uk/Electric ... tivePower/


Regards etc.


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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 7:07 pm 
Also this, which is rather hard to find!

http://www.nationalgrid.com/NR/rdonlyre ... _oct01.pdf


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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 8:40 pm 

Joined: Sun Jul 03, 2005 11:11 am
Posts: 6700
Passion_chorale, thank you, the second link is gold-dust.
Quote:
Overhead Lines and Underground Cables - Overhead lines and underground cables, when
operating at the normal system voltage, both produce strong electric fields and so generate
reactive power. When current flows through a line or cable it produces a magnetic field which
absorbs reactive power. A lightly loaded overhead line is a net generator of reactive power


Yet another nail in wind!

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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 9:56 pm 

Joined: Fri Feb 23, 2007 9:49 am
Posts: 307
passion_chorale wrote:
Hi, Bert,This link is quite a good starting point ...


Thank you. I will study this as time permits. I must say that I am most surprised at the apparent paucity of power engineers participating in the wind debate. Why is this? Presumably the near silence of the Wunch Of Bankers is out of self interest but how do power engineers benefit from the slide to a Banana Republic National Grid?

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 Post subject: Re: Confusion reigns
PostPosted: Mon Oct 27, 2008 11:51 pm 
I've been silently following this debate in the forum for some time, but have only now taken the decision to wade in. Principally i'm worried that we are treating power supply and demand as a purely scalar problem. What I mean by that is everyone has this assumption that provided supply can be matched to demand at all points in time, with a good safety margin, all will be fine. Thus the discussion of the huge expansion of wind farms boils down to a discussion about net and peak power supply and windspeeds and efficiencies of windmills etc. This 'angle' is easy for windfarm supporters to twist, because any lack in demand can simply be made up for by yet more windfarms in lots of different places.

Really, we need to look at it in a more fundamental way. I too am suprised at the lack of power engineers participating. What we really need is a cynical w*nker who came up on an apprenticeship through CEGB about thirty years ago. Someone with knowledge of how stuff used to be in the 'good old days', and somebody able to distinguish between the various newly announced methods of turning mud into gold. Kindof a Richard North in the electrical industry.

I'm in communications engineering research [not power engineering], and my total industrial experience adds up to about 40 weeks over a series of placements. Therefore you need to take some of this stuff with a shovel-full of salt. However, whether you deal in picoWatts or Gigawatts on a daily basis, Maxwell's Equations are still the same and so certain principles are transferable.

The first thing that needs to be understood is that the mathematics of electricity supply and distribution is complex. That's not a cliche; I mean 'complex' in the formal mathematical sense. Voltages and Currents on national grid power lines approximate sine waves, and so have amplitudes and relative phase differences at a constant frequency. This is rather different to the 'batteries n lightbulbs' approach with DC electricity.

In the 'batteries n lightbulbs' approach, the wires between the two can be considered to be purely resistive. Resistance is a 'real' quantity.

However, for a network the size of the national grid, you have to take account of the inductance and capacitance of the circuit elements as well, referred to as reactance. Reactance is still measured in Ohms, but it is a 'complex' quantity.

Therefore, the total impedance (complex) of an element of the national grid = resistance (real) + reactance (imaginary), measured in Ohms.

I don't want to get bogged down in the maths of complex numbers, but time spent on Wikipedia will show you that it's not all that hard. The real-life implication of this complex impedance is that reactances in the network cause the voltage and current waveforms to either lag or lead each other.

Simply stated; reactive power is unusable power associated with energy STORED in the network. Resistive power is associated with energy dissipated in the network, and that used at the destination.

That means that reactive power has to be considered in keeping an electrical network up and running, and balanced out where appropriate.

Big industrial companies are actually billed for the amount of reactive power they introduce to the network; it's an extra line on their electricity bill. The reason is that electricity companies rightly consider it to be the responsibility of the user to balance the power factor of their network before connecting it to the grid. Otherwise, you get these reactive power components which are considered a waste, because they cause more current to flow in the line than if they had been cancelled out.

I suppose, relating to wind farms, the point is that your supply network is going to be much more spread out and thus introduce much more reactive load to the network per W of windfarm, than if the network was just going straight to a nuke or coal plant. Not only is it a matter of who pays for the lines to remote areas in the first place, but who pays for the ongoing cost of balancing the power factor? Or is this inbuilt at the windfarm side?

What would be useful now is for a national grid engineer to come out of the woodwork.

More importantly, i suppose this backs up Richard's point re: food industry. Years ago, there would have been an entire technical bureacracy dedicated to researching and 'knowing' all this stuff. They would have presented policy options to the politicians and would have had the final say in whether or not they were practical. Now we just seem to have a weird bog of EU, power suppliers, the grid, politicians, and assorted tranzies. The EU, politicians and tranzies are simply sticking with their 'scalar' 'batteries n lightbulbs' analysis, formulating policy on the basis of that, and then giving it to the industry to chew on.


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 Post subject: Re: Confusion reigns
PostPosted: Tue Oct 28, 2008 1:05 am 

Joined: Sun Jul 03, 2005 11:11 am
Posts: 6700
It seems to me the only answer to these wind farm white elephants (besides stop subsidies NOW!) is to take the efficiency hit of using the wind-power to produce hydrogen by hydrolysis. So you only get 30% of the energy as gas, but that may improve with research. The main point is that gas pipelines aren't as complex as a grid and the transmission losses coupled with ease of storage and the usefulness of a carbon-free gas power station make the initial hit palatable. Certainly trying to connect the mega-farm they intended on the Isle of Lewes to the grid would have been extremely hairy, whereas keeping some gasometers charged would have probably made the isle power independent.

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 Post subject: Re: Confusion reigns
PostPosted: Tue Oct 28, 2008 1:15 am 
Doubt you would be able to justify a pipeline; volumes just wouldn't be great enough. Tankers possibly, but then what the hell is actually going to use Hydrogen in reality....perhaps we could use it to fill the balloons in a new generation of airships. For convenience, they would be called 'Hindenburgs'. What a salient metaphor.


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 Post subject: Re: Confusion reigns
PostPosted: Tue Oct 28, 2008 1:41 am 

Joined: Sun Jul 03, 2005 11:11 am
Posts: 6700
Quote:
what the hell is actually going to use Hydrogen in reality

A natural gas power station would need very little tinkering to run on hydrogen thus giving the grid planners an on-demand power-station with no EU carbon overheads. As for the Hindenburg being a block on using hydrogen, well, do you really want to enter that debate?
Welcome BTW I'm delighted you chose to join Dr. N and the Growlers.

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 Post subject: Re: Confusion reigns
PostPosted: Tue Oct 28, 2008 4:05 am 
Bert Rustle wrote:
passion_chorale wrote:
Hi, Bert,This link is quite a good starting point ...


Thank you. I will study this as time permits. I must say that I am most surprised at the apparent paucity of power engineers participating in the wind debate. Why is this? Presumably the near silence of the Wunch Of Bankers is out of self interest but how do power engineers benefit from the slide to a Banana Republic National Grid?


Well, there are not too many employers in the marketplace and all of those that are probably see the potential for more money via subsidies and carbon trading than they do from power generation alone.

So, your employer wants the 'developments' to happen and you the engineer, presumably interested in deploying your skills to solve challenges, have some challenges coming up and nowhere else to go.

Not really a 'phone a friend' situation for them, is it?


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